1. Field of the Invention
The present invention relates to electromagnetic metamaterials and more particularly to applications of such materials in electromagnetic cloaking.
2. Brief Description of Prior Developments
Current interest in electromagnetic metamaterials has been motivated by recent work on cloaking and transformation optics as is disclosed in J. B. Pendry, D. Schurig, D. R. Smith, “Controlling electromagnetic fields”, Science 312, 1780-1782 (2006); U. Leonhardt, “Optical conformal mapping”, Science 312, 1777-1780 (2006); A. Greenleaf, M. Lassas, G. Uhlmann, “The Calderon problem for conormal potentials—I: Global uniqueness and reconstruction”,Communications on Pure and Applied Mathematics 56, 328-352 (2003); and A. Alu, N. Engheta, “Cloaking and transparency for collections of particles with metamaterial and plasmonic covers”, Optics Express 15, 7578-7590 (2007), the contents of which are incorporated herein by reference. This interest has been followed by considerable efforts aimed at the introduction of metamaterial structures that could be realized experimentally. Unfortunately, it appears difficult to develop metamaterials with low-loss, broadband performance. The difficulties are especially severe in the visible frequency range where good magnetic performance is limited. While interesting metamaterial devices have been suggested based on non-magnetic designs as is disclosed in W. Cai, U. K. Chettiar, A. V. Kildishev, V. M. Shalaev, “Optical cloaking with metamaterials”, Nature Photonics 1, 224-227 (2007); W. Cai, U. K. Chettiar, A. V. Kildishev, V. M. Shalaev, G. W. Milton, “Nonmagnetic cloak with minimized scattering”, Appl. Phys. Lett. 91, 111105 (2007); and Z. Jacob, E. E. Narimanov, “Semiclassical description of non magnetic cloaking”, Optics Express 16, 4597-4604 (2008), the contents of which are incorporated herein by reference, the development of anisotropic magnetic metamaterials for the visible range would be highly desirable. Other limitations of “traditional” metamaterials in any portion of the electromagnetic spectrum are high losses and narrowband performance. These limitations may be once again illustrated using cloaking as an example. Despite considerable theoretical effort, there exist only a few experimental demonstrations performed in rather narrow frequency ranges. The first experimental realization of an electromagnetic cloak in the microwave frequency range was reported in a two-dimensional cylindrical waveguide geometry as is disclosed in D. Schurig, J. J. Mock, B. J. Justice, S. A. Cummer, J. B. Pendry, A. F. Starr, D. R. Smith, “Metamaterial electromagnetic cloak at microwave frequencies”, Science 314, 977-980 (2006), the contents of which are incorporated herein by reference. In addition, a plasmonic metamaterial structure exhibiting reduced visibility at 500 nm has been demonstrated as is disclosed in I. I. Smolyaninov, Y. J. Hung, C. C. Davis, “Two-dimensional metamaterial structure exhibiting reduced visibility at 500 nm”, Optics Letters 33, 1342-1344 (2008), the contents of which are incorporated herein by reference. In both experimental demonstrations, the dimensions of the “cloaked” area were comparable with the wavelength of the incident electromagnetic radiation, meaning that the shadow produced by an uncloaked object of the same size would not be pronounced anyway in these cases.
A need, therefore, exists for a way to further improve electromagnetic materials in using metamaterials.